In such a high pressure discharge lamp, the so-called foil seal structure, in which a base portion of an electrode axis is joined to a metallic foil buried in a sealing portion, is adopted as a sealing structure. In general, because the electrode axis of the electrode is made of tungsten while an arc tube is made of silica glass the sealing portion of the arc tube often breaks and/or is damaged due to difference in the thermal expansion coefficients. Especially, since a large amount of mercury (i.e. 0.15 mg/mm3 or more) is enclosed in a light emitting portion of such a high pressure discharge lamp used for a projector apparatus so that the mercury steam pressure turns into high pressure (i.e. 100 or more atmospheric pressure) at time of lighting, the above problem is much more serious.
Japanese Patent Application Publication No. 2008-529252 teaches technology in which grooves are formed on an axis of an electrode (core rod) to extend along in an axis direction thereof, in order to solve such a problem. FIG. 3A is a schematic view of the structure of a lamp according to a conventional example, and FIG. 3B is an enlarged view of an electrode. As shown in FIGS. 3A and 3B, two or more grooves 5, which extend in an axis direction of an electrode 2, are formed on an outer surface area of an electrode axis 21 provided in a discharge lamp 1, which faces a sealing portion 3 of the arc tube. In addition, the electrode axis 21 is connected to a metallic foil 4 inside the sealing portion 3. In the above-mentioned conventional technology, the surface coarseness in a circumference direction is made larger than that of a longitudinal direction thereof since two or more grooves 5 are formed on the electrode axis 21, thereby preventing breakage of the sealing portion due to the difference in the thermal expansion coefficient of the material (tungsten) of the electrode axis 21 and the material (silica glass) of the sealing portion 3.